direct measurement of swimming speeds and depth of blue marlin

J. exp. Biol. 166, 267-284 (1992) 2 6 7Printed in Great Britain © The Company of Biologists Limited 1992

DIRECT MEASUREMENT OF SWIMMING SPEEDS ANDDEPTH OF BLUE MARLIN

BY BARBARA A. BLOCK, DAVID BOOTH

The University of Chicago, Department of Organismal Biology, 1025 East 57thStreet, Chicago, IL 60637, USA

AND FRANCIS G. CAREYWoods Hole Oceanographic Institution, Department of Biology, Woods Hole,

MA 02543, USA

Accepted 30 December 1991

Summary

Acoustic telemetry was used to monitor the swimming speed, depth and watertemperature of three blue marlin (60 kg, 70 kg, 125 kg) and 165 h of continuousswimming speed data containing both sustained and burst swimming events werecollected. Measurements of swimming speed show that, while blue marlin arecapable of high speeds, they spend most of their time swimming slowly. The fastestsustained swimming speeds (80-120cms"') occurred during a 4-6h recoveryperiod immediately after tagging when marlin consistently swim at depths greaterthan 50m. Short bursts of speeds up to 225cms"1 were usually associated withchanges in depth. Slower swimming (15-25cms"1) occurred when fish werewithin 10 m of the surface. These velocities are similar to direct measurements ofswimming speeds of free-swimming sharks, seals and sea lions, indicating thatmany large aquatic vertebrates swim slowly to minimize energetic costs oftransport.

Introduction

Blue marlin, Makaira nigricans (Lace"pede), are members of the suborderScombroidei, a group that contains many large, commercially important marinefish such as tunas, swordfish, marlin and spearfish. The genus Makaira includes thelargest living teleosts with reported specimens weighing over 800 kg (Mather,1976). Marlin are solitary pelagic fish that make yearly migrations of thousands ofkilometers across ocean basins (Squire, 1985, 1987; Davie, 1990). The mostnotable long-distance return is that of a 58kg black marlin, Makaira indica,released off Baja, California, and recaptured 10000 km from the point of releaseoff New Zealand by a Japanese longliner (Squire, 1985). They are highly adaptedto a pelagic mode of life, having numerous morphological features that enhancelocomotor performance. Body surfaces are characterized by extraordinary stream-Key words: telemetry, speed, locomotion, blue marlin, Makaira nigricans.

268 B. A. BLOCK, D. BOOTH AND F. G. CAREY

lining, pectoral fins are swept back and have large surface areas for lift, while ahigh-aspect-ratio tail provides thrust. Many of their locomotory adaptations arethought to be related to the high burst speeds of these fish. The large size andpelagic habitat of blue marlin make them difficult to study. Only recently, with therapid world-wide depletion of blue marlin stocks, has attention focused on thebiology and conservation of this species. Laboratory-based research on bluemarlin and other istiophorids (marlins, spearfish, sailfish) has rapidly increased inthe past decade (for reviews, see Davie, 1990; Stroud, 1990) and has revealednumerous physiological and morphological specializations for a nomadic lifestylein the open sea. However, because of the difficulties of studying free-ranging fishin the open ocean, there is little knowledge about what these fish do in the wild.

The powerful and acrobatic movements of blue marlin caught on hook and linehave long captured the imagination of fishermen, and it is well established thatthey can explosively strike a bait trolled at high speeds (800cms"1, F. Rice,personal communication) but no direct speed measurements have been made.Blue marlin are commonly assumed to be among the fastest of all fishes andswimming speeds as high as 2000cms"1 have been estimated (Walters, 1962;Davie, 1990). The power required to reach such velocities is extraordinarily high(Johnston and Salamonski, 1984; Hebrank et al. 1990). These bursts are thought tobe powered by white muscle fibers, which constitute the bulk of the skeletal musclemass.

By placing speedometers on blue marlin, we were able to obtain a continuousrecord of swimming speed and relate variations in speed to changes in depth andbehavior. We present here the first direct measurements of swimming speed forblue marlin and show that, while capable of high speeds, they spend most of theirtime swimming slowly.

Materials and methodsMultiplexed acoustic transmitters capable of sending pulse-encoded data from

depth, water temperature and speed sensors were attached to three blue marlinestimated to weigh 70, 60 and 125 kg. The three fish studied here (marlin 4, 5 and6) were part of a larger study in which six marlin were tracked off the Kona coast ofthe island of Hawaii (Block et al. 1992).

Body length of marlin was difficult to estimate accurately because fish werecontinually in motion during the tagging procedure. Restraining them for accuratemeasurement would have led to unacceptable stress and injury. The length of thelargest fish in our study (225 kg) was estimated as 1.8 m (lower jaw to fork length)from photographs taken during tagging. Masses of fish were estimated by theprofessional captains of sportfishing boats who are remarkably accurate with theirestimates (±5 kg). It should be noted that estimating body length from body massis complicated by the inability to determine sex externally in blue marlin: there areconsiderable differences in the mass-length relationships of males and females.

Swimming speeds of blue marlin 269

'lIlllllJlIllllllffllllllljlLlllSjjljJiljl

Fig. 1. Speedometer transmitter. The thermistor, pressure sensor and cable to thespeed rotor are at the front (right) and the acoustic transducer on the rear (left). Asmall dart that holds the transmitter to the fish is attached to the aluminum eye. Theroll of wire is removed before use.

Marlin estimated to be larger than 140 kg in Kona waters are assumed to be female(Hopper, 1990).

A transmitter with a speedometer is shown in Fig. 1. The multiplex transmitterswere 18.0 cm long, 4.0 cm wide and 2.2 cm thick. They weighed 190 g in air and 78 gin sea water. The speed sensor was a 5 cm diameter, 10 cm pitch, plastic propellor(Octura Models 2050, Skokie, IL) containing a magnet which activated a magneticreed switch. Circuitry in the transmitter divided the switch closures by 16 to give apulse repetition rate for the speedometer between 0.5 and 2 Hz at the swim speedswe observed. The rotor was attached to a shaft mounted in a streamlined frontalpiece cast from buoyant syntactic foam. A flexible 3 mm diameter 35 cm long cableran from the transmitter to the rotor assembly, where it was attached asymmetri-cally to the lower side of the frontal piece. The asymmetric shape, and thebuoyancy of the foam, caused the rotor assembly to plane away from the boundarylayer surrounding the fish. The cable allowed the rotor to align with water flowregardless of the orientation of the transmitter. We observed this device onmarlins 5 and 6 and in both cases the rotor was trailing above and behind the point


of attachment and aligned with the direction of flow. The speedometer, calibratedby towing in a flume at known velocities, had a linear response over a range ofspeeds from less than 20cms"1 to 210cms~1, the maximum possible in our flume.Although the system could record faster speeds, such speeds were not encoun-tered during experiments. The transmitter also contained a thermistor mounted onan aluminum stub to measure water temperature and a strain gauge pressuresensor (Keller PA-2-20) for depth. Data was time-multiplexed by the transmitterclock so that 32 s of information on speed, 24 s of depth and 8 s of watertemperature data were broadcast in sequence over a continuously repeating 64 scycle. A similar crystal clock on shipboard stayed synchronous with the transmitterclock and allowed the computer to identify the parameter currently beingbroadcast. The data collection system is described by Block et al. (1992). Duringdata analysis, a mean was calculated for the parameters of interest for each dataperiod within a 64 s cycle.

ResultsThe blue marlin were followed for periods of 25-120h. Analysis of depth

recordings from three marlin in the depth/speed study, as well as from three otherfish fitted with depth/temperature transmitters, indicates a preference among bluemarlin for the mixed layer, with the fish swimming predominantly above thethermocline (Figs 2 and 8). Figs 3-5 show continuous swimming speed records forthree blue marlin. Similarities in their swimming behavior are apparent. Immedi-ately after tagging, marlin 5 and 6 entered into a stereotypic recovery periodlasting 4-6 h (Fig. 2; Holland et al. 1990), in which fish descended to 50-100 m andswam at elevated speeds (80-100cms"1). Swimming during this recovery period(Fig. 6) is presumed to be powered predominantly by aerobic red muscle fibers.

Blue marlin 5. 4-5 August 1989

200

Fig. 2. Depth record of marlin 5 superimposed on a 1°C isotherm plot drawn fromexpendable bathythermograph casts. The dark bar indicates night. The recoveryperiod lasted until sunset, 5.5 h after release.


Fig. 3. Swimming speed record of marlin 4 off of Kona, Hawaii. Elevated swimmingspeeds are associated with periods at depth. The short rapid increases of speed areusually associated with rapid dives. This marlin was killed by a shark at 05:15 h.


12

Fig. 4. Swimming speed record of marlin 5.

When in near-surface waters (<10m), marlin always display much slowerswimming speeds (<25 cm s"1). Because blue marlin spend over 50 % of their timein the top 10 m of the water column (Holland et al. 1990; Block et al. 1992), most ofthe time these fish were travelling at low speeds.

The frequency distributions of swimming speeds for the cumulative track time ofall three blue marlin are shown in Fig. 7. The most striking result is the relativelyslow speeds at which marlin normally swim. For over 97 % of the time, swimmingspeed was less than 120cms"1 (0.7BLS"1 for marlin 6, where BL is body length),and the maximum speed we observed was less than 225cms"1. Typically, longperiods of swimming at speeds of less than 30cms"1 were punctuated by shortincreases in speed up to 225cms"1. Periods of elevated speed were oftencorrelated with movement of the fish to greater depths (see Figs 8 and 9).

The following is a brief description of the track of each fish tagged with aspeedometer. Marlin 4, 70 kg, (Fig. 3) was exhausted after capture on hook and



Fig. 5. Five-day swimming speed record of marlin 6. Dark bars indicate night. Thisfish showed a diel record with slow periods of swimming at the surface during the dayand periods of faster swimming during the night. Days three, four and five show cleardifferences between the average daytime and nighttime speed.


r- 150-

21

Fig. 6. Comparison of the elevated sustainable swimming speed of three marlin. Thisprobably represents aerobic swimming powered by the red muscle mass. The elevatedsustainable speeds are strikingly similar for these three fish of different body size.

line. The fish was released after being towed to irrigate the gills, which facilitatedits recovery from being tagged. The marlin began a short period (<2h) ofsustained swimming (75-100cms""1) but remained close to the surface (<20m)and headed in a south-westerly direction. The behavior of this marlin wasdistinctly different from that of all other fish tracked. It stayed close to the surfacewhile swimming away from the point of release and shortly after release its speedgradually slowed to a range of 55-85 cm s"1. During the early hours of themorning, the fish began changing depth erratically and rapid increases of speed(170-215 cm s~J) were associated with the beginning of each descent. At 05:00 h,


25

Speed (cm s ')

Fig. 7. Summary of the speed data for all three marlin indicating the amount of timespent at any given speed. Speed was divided into 20cms"1 bins and the percentage oftime spent at each speed was calculated for the cumulative data from all tracks.

the marlin displayed a rapid acceleration (Fig. 3) while simultaneously descendingto 60 m depth. This was followed by the abrupt stopping of the speedometer signal,although the depth and temperature channels continued to transmit data. Asreported by Block et al. (1992), we associate this event with a shark attack on thisfish.

Marlin 5, 60kg, struggled at the side of the boat for several minutes duringtagging but seemed in fair condition upon release. Immediately following tagging,this fish began a recovery period swimming offshore at a speed of 100cms""1,which it sustained for 6h. It remained below 50m depth throughout this periodthen rose to near the surface and slowed to speeds between 12 and 25 cms"1. Slowswimming while at the surface was punctuated by short descents during which thefish swam at speeds as high as 155cms"1 (Fig. 4). Such increases in speed weregreatest when the fish descended below 50 m. This track ended abruptly as thetransmitter sank to the bottom at a constant speed. While sinking, speed from thespeedometer registered 46 cms"1 while change of depth with time indicated aspeed of 45cms"1, an excellent in situ check on speedometer calibration.

Marlin 6, 115 kg and 1.8m in length, was brought to the boat after a 15minstruggle and was released in excellent condition. During a 6 h recovery period, thisfish maintained a consistent speed of 100±18cms~1

(S.D.) while swimming atdepths of 50-100m. For a short time (2.5 h), the marlin slowed down to between20 and 50cms"1 and came to within 10m of the surface. This was followed byanother 2h of swimming at elevated speed (100cms"1). In contrast to the otherfish observed, marlin 6 showed a clear diel variation in both depth movements andspeed (Figs 5,8). During daylight hours this marlin was usually near the surface in27°C waters, swimming at less than 50cms"1. It made occasional short, 20-30s



Q.

Q

50-

100-

150-

200

- 18°C

6 12Blue marlin 6. 9-10 August 1989

18 24

Fig. 8. Depth record of marlin 6 superimposed on 1 °C isotherms from day three of thetrack. There is a clear correlation between depth and speed. While at the surface themarlin swims slowly, when at depth the sustained speed of the fish increases.

elevations of speed associated with dives to 70 or 100 m. In the evening,approximately at dusk, there was a period of sustained swimming, 80-100 cms"1,similar to that seen at the beginning of the track. During the night this fish madenumerous dives to depths between 50 and 100 m, which were again associated withincreases in speed up to 150cm s~L. There was a significant difference (P<0.001;Student's /-test) between average speed during the day (500ns"1) and during thenight (67 cm s"1). This was correlated with a significantly different (P<0.05) depthdistribution of the fish between day and night.

In Fig. 9 the relationship between speed and depth for marlin 6 is examined overa finer time scale. Three observations are readily made: (1) while at the surface themarlin is travelling at a relatively slow speed, less than 50cms"1, (2) there is anincrease in speed to 135cms"1 on leaving the surface, and (3) while at depth,speed remains elevated (approximately 100cms"1) and only returns to slowervalues upon ascent to the surface. This pattern of descent and ascent is typical ofall marlin observed. There is a clear relationship between depth and swim speed.To examine this more critically, we made a scatterplot of speed, averaged over 32 s


A

Blue marlin 6. 14:12 h to 15:42 h. 8 August 1989O-i r 150

in

30 60

Time (min)

Blue marlin 6. 17:30h to 18:00h, 7 August 1989

90

50

15 20 25 30 35

Time (min)

40

Fig. 9. (A,B) Speed during diving in marlin 6. The solid line shows depth, the dottedline speed. Marlin swim slowly while in near-surface waters. Accelerations with depthduring diving suggest that the marlin is positively buoyant while at the surface. When atdepth, the marlin increases speed.

intervals, against the average depth in the consecutive 24 s depth interval(Fig. 10A). While at depth the fish chose to swim at one of two distinct speeds,either SS^Ocms"1 or 80-95cms"1. A similar profile of depth with speed wasobtained for marlin 5 (Fig. 10B). Marlin 5 also increased speed at depth, but hadonly a single speed range at depth, 75-100cms"1, in contrast to the dual speedrange of marlin 6.


ABlue marlin 6. 6-11 August 1989

200

0 25 50 75

Depth (m)


Depth (m)

Fig. 10. (A) Recruitment of swimming speeds with depth from marlin 6, a two-'gear'pattern, and (B) marlin 5 with one sustained speed. It should be noted that marlin 5was tracked for a considerably shorter period than was marlin 6 (25 h versus 120 h).

Discussion

How fast do marlin swim?

Istiophorid fish are thought to be among the fastest-swimming fishes and havebeen estimated to swim at speeds of up to 3610 cm s - 1 (Table 1). In this study,swimming speed has been measured directly in three free-swimming blue marlin.


Table 1. Maximum reported swimming speeds of pelagic fish

Fish

Black marlinBlue marlinYellowfin tunaWahooSailfishBarracudaBlue marlinBlue marlinBlue marlinStriped marlinSwordfishBlue sharkBluefin tuna

Swimming speed

(cms"1) (kmh"1

361020802080214030001220220170225180225200150

* Tracking, speed calculated

130757577

10844

868.16.58.175.4

) Method

EstimateEstimateRod and reelRod and reelEstimate-Tracking*TrackingSpeedometer)"TrackingTrackingSpeedometerTracking

Reference

Walters (1962)Walford (1937)Walters and Fierstine (1964)Walters and Fierstine (1964)Lane (1941)Blaxter and Dickson (1959)Yuen et al. (1974)Holland et al. (1990)This studyHolts and Bedford (1990)F. G. Carey (unpublished results)Carey and Scharold (1990)F. G. Carey (unpublished results)

from translation of position over time.t Speedometer, speed measured by speedometer telemetry.

The highest continuous swimming speed (speed maintained for longer than 0.5 h)was almost the same (100 cm s"1) for the three marlin although the mass of the fishvaried by a factor of two. The most striking result of these direct speedmeasurements from free-swimming fish is the slow speeds at which blue marlinswim. In the literature on scombroid fish there are frequent references to these fishas swift swimmers (Davie, 1990), and our results are in sharp contrast to the veryhigh swim speeds that have captured the imagination of many writers. The factthat they can strike a bait trolled at 800 cm s~l leaves no doubt that they arecapable of short bursts of high-speed swimming, but we saw no such rapidmovements in 160 h of speed observations. High-speed swimming and agility musthave a place in the repertoire of activities displayed by blue marlin but, during thesummer near Hawaii, such behavior is rare. There were only infrequent 10 to 30 sbursts of speeds exceeding 200cms"1. Short periods of fast swimming areundoubtedly important for catching certain prey, and hence important in terms ofsurvival, but the characteristic mode of swimming we observed for this species wasquite slow.

Our swimming speed values for blue marlin are similar to those recorded forfree-swimming blue sharks {Prionace glaucas) using speedometer transmitters.Blue sharks ranging from 2.2 to 2.8 m in length commonly swim at speeds of40-70cms"1 for periods of many hours (Carey and Scharold, 1990). The fastestblue shark speed was about 200cms"1 in short bursts. Similarly, a 1.8m makoshark {Isurus oxyrinchus) swam at an average speed of 90 cm s"1 over a 24 h periodand had a maximum speed of ISOcms"1 (F. G. Carey, unpublished data). Weihs(1981) also found carcharhinid sharks (1.5-3 m in total length) swimming in an


aquarium to have swimming speeds ranging from 60 to 80cms"1. Although nodirect measurements are available, swordfish and bluefin tuna speeds, determinedfrom translation of position during tracking, fall within the range of swimmingspeed observed for marlin (Table 1). Swimming speeds measured with speed-ometers on marine mammals of comparable body size to these fish are similarlyslow, 100-200cms"1 (Fedak and Thompson, 1990; Ponganis etal. 1990). Whenmoving over long distances in the open ocean, large animals travel slowly tominimize the cost of transport.

Estimates of sustained and maximum swimming speeds of pelagic fishes varywidely in the literature. The fastest speeds recorded were obtained by Walters andFierstine (1964), who measured the speed at which line ran off a fishing reel duringa strike. They found that wahoo (Acanthocybium solanderi) and yellowfin tuna(Thunnus albacares), species much smaller than blue marlin, accelerated to2100cms"1 (19BLs"') within the first 5s after striking a bait. Magnuson (1978)has compiled the most comprehensive speed data for comparing continuous versusburst swimming in various scombroid fishes. These data were obtained by a varietyof techniques ranging from aerial photographs to acoustic tracking. Continuousswimming speeds ranged from 0.3BLs"1 in wahoo to 1.6BLS"1 in bluefin tuna.Reported burst swimming speeds of scombrids range widely from 8BLs"1 in thebonito to 27BLs"1 in yellowfin tuna. Speed estimates from cinematography offeeding tuna were in the range 9-14BLs"' (Yuen, 1966). Some of Yuen's recordsare faster than any speeds from tracking, but even the speeds of these activelyfeeding fish are an order of magnitude lower than many of the estimated speedscited in the literature.

Speeds measured from translation of position over time while tracking bluemarlin range from 20 to 222cms"1 (Yuen et al. 1974; Holland et al. 1990; Block etal. 1992). Comparison with other scombroid data is difficult owing to the absenceof length measurements on these fish which, as explained earlier, are difficult toobtain from free-ranging fish. The speed estimates from tracking other billfishes,striped marlin (Tetraptums audax) and swordfish (Xiphias gladius), are similar tothose for telemetered data from marlin (Holts and Bedford, 1990; Carey andRobison, 1981). Such speeds are an order of magnitude lower than most of thevalues reported in earlier literature (Table 1). Tag and recapture programs in boththe Pacific and Atlantic have shown that blue and black marlin are capable ofyearly transoceanic journeys covering thousands of kilometers (Squire, 1974,1985,1987). A black marlin, released off southern California, was recaptured 3 monthslater off Peru, a journey requiring a straight-line speed of at least 55 km day"1

(64 cm s"1). Mather (1976) reported a speed of 76 cm s"1 for bluefin tuna migratingfrom the Bahamas to the latitude of Bergen, Norway. These tag recapture datagive minimum estimates: the fish had to move at least this fast, although they mayhave moved faster.

Theoretical and empirical data on fish locomotion have suggested that maxi-mum cruising speed will increase in fish as BL043 (Weihs, 1977, 1981). Wardleet al. (1989) also predicted that cruising speeds in large aquatic vertebrates will


decrease with increasing size as a result of the large increases in drag that occurwhen the flow in the boundary layer changes from laminar to turbulent. Perhapsour surprise at the slow speed of marlin lies in the fact that the literature as well aslaboratory measurements of swimming speed and performance in small scombroidfishes suggest that we should expect higher sustained swimming speeds. Moststudies on fish swimming performance have been made on relatively smallspecimens (<3kg) swimming in confined water tunnels. It is not clear how thesemeasurements scale with body mass. Thus, their relevance in predicting thebehavior of fishes several orders of magnitude larger is questionable. Onlyrecently, with the construction of a large swimming tunnel (Graham etal. 1990),has it been possible to work on larger fishes. Although this new tunnel provides anunparalleled situation for studying several parameters of performance in oceanicfishes, natural behavior can only be studied in the field where the fish sets the limitsfor its activities. The speeds observed by telemetry from free-swimming fish aredetermined by the fish themselves. With advances in recorder and telemetrytechnology, the ability to study fish in the wild will increase, allowing betterintegration between laboratory and field data.

Recruitment of swimming speeds

There is a striking relationship between depth and speed for marlin 6(Fig. 10A). These high and low 'gears' may be analogous to gait changes interrestrial tetrapods, and thus represent fixed neural patterns of fiber recruitment;red fibers being recruited during slow sustained swimming (<50cms~J) and redplus fast-twitch oxidative fibers being recruited during faster sustained swimming(>80cms~1). The red slow-twitch oxidative fibers constitute only 5-6% of thetotal muscle mass (Davie, 1990) and are located in numerous small bundles spreadthroughout the peripheral region of the epaxial musculature. Most of the epaxialmuscle consists of white muscle, which is a mosaic of at least two fiber types, largefast-twitch glycolytic fibers and smaller fast-twitch oxidative fibers (B. A. Block,unpublished observation). Only one preferred sustained swimming speed(100cms"1) is apparent for marlin 5 (Fig. 10B). A preferred speed may representthe most efficient speed for irrigating the gills and extracting oxygen at minimalcost, during periods when the fish is covering large distances.

Economy of transport

In the warm seas inhabited by blue marlin, food resources are often unevenlydistributed. Hence, minimizing energy expenditure while swimming long distancesbetween food patches would be advantageous. Energy required for transportincludes the energy for moving a given distance plus the energy required forroutine metabolic functions. The most energy-efficient speed (i.e. minimal energyexpended per unit distance travelled) will be near the low end of the speed range(Weihs, 1984). Many billfishes and other scombroids travel thousands of kil-ometers during extensive seasonal migrations (Squire, 1974). Several blue marlintagged in the Western Atlantic off North Carolina have been caught along the


Ivory Coast of Africa. Large bluefin tuna feed in rich temperate waters during thesummer and migrate long distances to spawn in tropical areas in winter. The timescale for these migrations is one of seasons. Travelling at 114cms"1 (2knots) issufficient to take a fish 6400 km (4000 miles) in the course of one season. The factthat the cruising speeds of all these fish are exceedingly low suggests that slowswimming is a way to minimize the cost of locomotion over long distances. Thishypothesis may extend beyond scombroid fishes. Salmon, eels, cod and numerousother species make extensive migrations. When tracked by telemetry, such fishalso travel at low speeds, ranging between 25 and 100cms"1 (Madison et al. 1972;Tesch, 1979). Although the swimming speeds of salmon recorded by telemetrywere generally comparable to values from other salmonid studies, Madison et al.(1972) noted that the average speed for sockeye salmon was well below themaximum values recorded in an experimental flume.

Because of the high-aspect-ratio tail and the convergence in external design withtunas, marlin were, until recently, thought to be thunniform swimmers (Lindsey,1978 ). Video analysis of free-swimming marlin and studies of the biomechanics ofthe backbone indicate that, unlike tunas, these fish are quite flexible and swim witha sinuous shark-like motion (Hebrank et al. 1990). Marlin are subcarangiformswimmers, the propulsive force being delivered by the caudal fin and thedisplacement of water during the side-to-side undulatory movements associatedwith swimming. This form of swimming may limit the maximum sustainedswimming speeds of istiophorids and the stiffening of the body axis as seen inthunniform swimming may allow higher sustained swimming speeds. Directspeedometer telemetry on tunas of similar body size to the marlins should be usedto compare speeds directly between these large fish that use different kinematicforms of swimming. While speeds of tunas obtained from translation of positionover time during telemetry experiments are comparable to those of marlin, itwould be interesting to know whether there is a significant increase in sustainablecruising speed afforded by the thunniform mode of locomotion.

Buoyancy

Blue marlin have an unusual swim bladder in the form of a thin-walled, multi-chambered sac extending from the level of the first pectoral fin to behind the firstanal fin. Observations of marlin 'finning' near the surface with the dorsal andcaudal fins high above the water show that the bladder is large enough for the fishto be positively buoyant near the surface. Problems associated with buoyancy arerelated to swimming speed in two ways. First, the observation that marlin swim athigher speeds when at depths of 50-100 m may be related to the collapse of theswim bladder at these depths. As the fish becomes negatively buoyant whenswimming deeper, it may be generating the required additional lift with its pectoralfins by swimming faster. Second, the frequently observed increase in speed at thebeginning of a descent may result from the difficulties in getting a positivelybuoyant fish away from the surface. Elevation of swim speed while descending andat depth is not observed in sharks (F. G. Carey, personal observation), which are


negatively buoyant at all depths (Bone and Roberts, 1969). The buoyancyprovided by the large swim bladder allows blue marlin, swordfish and perhapsother istiophorids to swim slowly when near the surface and thus conserve energy.

Warm muscles and sustained cruising speeds

The ability of scombroid fishes such as tunas and lamnid sharks such as the makoto conserve metabolic heat and raise their muscle temperature has often beenlinked to higher sustained swimming speeds (Beamish, 1978; Magnuson, 1978;Webb, 1990). However, warmer does not necessarily mean faster. Ectothermicmarlin operating at temperatures ranging from 17 to 27°C were observed to havefaster sustained swimming speeds than those of warm-bodied mako sharks ofsimilar body size (B. A. Block and F. G. Carey, personal observation).Ectothermic wahoo and endothermic yellowfin tuna of similar body size bothachieved the same high sprint speeds (Walters and Fierstine, 1964). Theassumption that higher body temperature in warm fish is associated with highersustained swimming speeds and sprint speeds needs to be tested. Endothermicbluefin tuna may travel for days at 100-200cms"1, speeds that are within the rangereported here for ectothermic marlin. Perhaps the most obvious advantage of awarm body temperature is unimpeded performance at all ambient water tempera-tures, giving warm fish the ability to forage in the cold waters beneath thethermocline, a foraging area that ectothermic fish are reluctant to enter (Block,1991).

It remains possible that there is a direct link between the thunniform mode oflocomotion and endothermy. Marlins, utilizing subcarangiform locomotion, havethe generalized primitive percomorph condition of laterally placed red muscle.Concomitant with the stiffer locomotory pattern of 'thunniform' swimming is theinternal localization of the red muscle that powers sustained swimming. In futurestudies, it would be useful to explore the relationships between swimmingperformance, red muscle positioning, speed and body temperature.

This work was supported by a grant from the Billfish Foundation and by grantsDCB-8958225 to B.A.B. and OCE-8811421 to F.G.C. from the National ScienceFoundation. We wish to thank the Pacific Ocean Research Foundation, HawaiianBig Game Fishing Club, for their support. This work would not have been possiblewithout the skill and patience of Captains Mike Hind and Stu Miyamoto of theF/V Heola, and Captains Freddy and McGrew Rice who provided the opportunityto tag blue marlin 6 aboard the F/V Ihu Nui. Tagging of marlin was also madepossible by the anglers and captains of the 1989 Kona Hawaiian and HawaiianInternational Billfish Tournaments. Data processing was expedited by the carefulwork of Karen Mazurkiewiez and Sue Conova. Many others helped to bring thisproject to fruition and we are indebted to the following for their effort andencouragement: Oliver Brazier, Otis and Janet Butler, Peter Fithian, John Longand Winthrop Rockefeller.


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D. J. Randall), pp. 101-187. New York: Academic Press.BLAXTER, J. H. S. AND DICKSON, W. (1959). Observations on swimming speeds offish. J. Cons.

Perm. int. Explor. Mer. 24, 272-479.BLOCK, B. A. (1991). Endothermy in fish: Thermogenesis, ecology and evolution. In

Biochemistry and Molecular Biology of Fish, vol. I (ed. P. W. Hochachka and T. Mommsen),pp. 267-311. Amsterdam: Elsevier Scientific Press.

BLOCK, B. A., BOOTH, D. T. AND CAREY, F. G. (1992). Depth and temperature of the bluemarlin, Makaira nigricans, observed by acoustic telemetry. Mar. Biol. (in press).

BONE, Q. AND ROBERTS, B. L. (1969). The density of elasmobranchs. / . mar. biol. Ass. U.K. 47,754-756.

BROCK, R. E. (1984). A contribution to the trophic biology of the blue marlin {Makairanigricans, Lacdpede, 1802) in Hawaii. Pac. Sci. 38, 141-148.

CAREY, F. G. AND ROBISON, B. H. (1981). Daily patterns in the activities of swordfish, Xiphiasgladius, observed by acoustic telemetry. Fish. Bull. 79, 277-291.

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direct measurement of swimming speeds and depth of blue marlin

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